Diatomaceous earth has been used in the food field, particularly in the process of treating fermented liquids, such as the removal of yeast fluids, solid matter, colloids or the like from beer or wine after fermentation. However, the use of diatomaceous earth incurs several problems including the unconfirmed safety of diatomaceous earth, the disposal of used diatomaceous earth, which cannot be incinerated, and the high disposal cost for the large amount of diatomaceous earth required. Accordingly, in recent years, hollow fiber membrane-type devices of advantageously small body size, such as ultrafiltration membranes or microfiltration membranes, have been attracting attention as devices to be used for the fermented liquid treatment.
In the treatment of fermented liquids such as wine, beer or the like using a hollow fiber membrane module, the fermented liquid is generally refined by way of cross-flow filtration, in which the fermented liquid is filtrated from inside to outside of the hollow fiber membrane by supplying the fermented liquid to a hollow portion of a hollow fiber membrane at a high flow amount under a high pressure of about 1 to 1.5 bar. The hollow fiber membrane used in this filtration must have a high filtration performance per unit area thereof, reduced clogging of the membrane during the filtration, i.e., reduced time-dependent degradation of the membrane. The membrane performance is also required to be sufficiently recovered by washing.
Patent Literature 1 discloses a fermented liquid treatment method using a hollow fiber membrane in which the processing efficiency is increased by the usage of an improved filtration system and an improved filtration method. However, this method is directed to the system and method of filtration, and is completely silent about the performance or characteristic of the hollow fiber membrane, which has the biggest influence on the performance of the fermented liquid filtration.
Further, Patent Literature 2 discloses a technique for preventing flavor changes during fermented alcohol filtration such as wine filtration. In this technique, the membrane filtration module is washed with an alcohol aqueous solution before an alcoholic beverage is supplied to the module, thereby preventing flavor changes caused by a sealing material, a potting material, or a membrane material. However, this method is directed to preprocessing of a membrane before the filtration of fermented liquids, and is silent about the performance or characteristics of the hollow fiber membrane.
Patent Literature 3 discloses a method of preventing membrane clogging during filtration of food such as beer, wine, liquor, soy sauce, fruit juice and the like, by using a porous complex structure. This porous complex structure is obtained by providing a water-soluble polymer having been processed to be water-insoluble in the porous voids of a polytetrafluoroethylene porous body (PTFE, hereinafter). However, this method requires crosslinking treatment to make the polymer water-insoluble after the water-soluble polymer is permeated into the porous voids of a PTFE, thereby causing some complications in the process. Moreover, using the expensive PTFE increases the production cost.
Furthermore, Patent Literature 4 discloses another filtration membrane for the filtration of beverages such as fruit juice or wine. This membrane is produced by adding a C2-12 acid anhydride to a spinning solution to provide a sharp pore size distribution and durability to the membrane, thereby improving the productivity. However, because of the acid anhydride contained in the spinning solution, this method incurs problematic degradation of the membrane material due to oxidization while the membranes are stored.
In general methods, the pore diameter is increased so as to improve permeability of a hollow fiber membrane; however, this tends to decrease the fractionation performance and the strength at the same time. A hollow fiber membrane is roughly classified by its membrane structure, to symmetrical membranes in which the pore diameters are substantially constant in the direction along the membrane thickness of the hollow fiber membrane; and asymmetrical membranes in which the pore diameters continuously or discontinuously change, and thus differ at the inner surfaces, inside portions and outer surfaces of the membranes. Among them, the symmetrical membranes exhibit a large resistance to the flow of the fluid at their entire thickness portions when used for filtration; therefore, it is difficult to achieve a large flow amount. This also tends to cause clogging of the membrane due to the solutes (substances to be removed). The removal of the unwanted substance by the filtration from the liquid is ensured by the surface layer effect related to the pore diameter of the membrane surface, and the membrane depth effect related to the thickness portion of the membrane. Separation mainly relying on the membrane depth effect is expected to achieve sharp fractionation performance; however, it is difficult to achieve a larger flow amount thereby because this separation requires a membrane to have a certain thickness. Further, this type of membrane also has a defect in that the flow amount tends to decrease with time because of the clogging of the membrane due to a substance to be removed. The contribution of the membrane depth effect is relatively large in the above-described symmetrical membrane, and thus, this defect is considered to be easily revealed in the symmetrical membrane. Further, it is a general practice to increase the density of one of the membrane surfaces in which the fluid comes through so as to enable the performance of a hollow fiber membrane to be recovered by washing; however, this also results in a disadvantageous decrease in the filtration flow amount.
Patent Literature 5 discloses a semipermeable membrane with a multilayer structure, which comprises a separation layer A comprising a ε-caprolactam soluble polymer and having a molecular cut-off of from 500 to 5,000,000 Daltons, a support layer B having a hydrodynamic resistance negligibly small as compared with those of the layers A and C, and the layer C having pores with diameters larger than those of the pores of the separation layer A and smaller than those of the pores of the support layer B. Described as the subject matter to be achieved by this technique is to provide a membrane that enables accurate control of the molecular cut-off and hydrodynamic permeability in which the hydrodynamic permeability is accurately controlled without relation to the molecular cut-off. This enables production of a membrane having a designated molecular cut-off and having a low, medium or high permeability as required. However, the strength, membrane performance sustainability and membrane performance recoverability of this membrane are not considered in this prior art.
Patent Literature 6 discloses a membrane characterized such that the membrane has micropores with a pore diameter of 500 nm or less at a layer in the proximity of the membrane inner wall surface and a micropore distribution at its cross section in the membrane thickness direction, and whose distribution has at least one pore with a maximum diameter of a specified value. This technique is substantially directed to a medical membrane excellent in biocompatibility, intended to inhibit a high molecular weight protein from infiltrating the membrane and to decrease a contact area of the membrane to the high molecular weight protein to thereby improve the biocompatibility, by forming a dense inner surface which is to contact blood. Further, the dense structure in the proximity of the outer surface of the membrane, following the maximum pore diameter portion of the section of the membrane, is intended to inhibit infiltration of an endotoxin fragment from the outer surface of the membrane. Therefore, the dense-coarse-dense structure of the membrane is provided as a required structure for a blood-treating membrane for its substance-removing ability, biocompatibility and inhibition of infiltration of endotoxin. Patent Literature 6 is silent about any relationship with other properties such as membrane performance sustainability or membrane performance recoverability after washing.
[Citation List]
[Patent Literature]    Patent Literature 1: Japanese Unexamined Patent Literature No. 2002-525196    Patent Literature 2: Japanese Unexamined Patent Literature No. 2006-136278    Patent Literature 3: Japanese Unexamined Patent Literature No. 1993-301034    Patent Literature 4: Japanese Unexamined Patent Literature No. 2005-515881    Patent Literature 5: Japanese Unexamined Patent Literature No. 1999-506387    Patent Literature 6: Japanese Unexamined Patent Literature No.